The present invention relates to etching of materials.
In many modern industrial processes it is necessary to etch intricate and accurately defined patterns in a material. The etching may be confined to within the material or the etching process may be continued until the material is etched through, thereby to expose an underlying material. In many industrial applications such as, for example, the fabrication of electronic, optical or optoelectronic devices, the etching steps form a very critical phase of device fabrication, as the etching process usually dictates the accuracy to which the devices are ultimately manufactured.
The use of etch masks, such as photo masks or shadow masks, is well known in such fabrication techniques. Hence, these processes will not be described in detail in the context of the present invention. However, in recent years, new forms of devices have been proposed for which such conventional etching techniques present severe process concerns. Furthermore, the etching of relatively long but extremely narrow lines has, for a long period of time, presented severe fabrication difficulties as it is very difficult to produce mechanically robust etch masks which will provide the required definition in the finished product.
Other concerns are also known to exist with known etching techniques. For example, in certain processes where it is required to etch through a layer of material to expose a pattern of an underlying material, such as a substrate, the surface of the substrate usually exhibits a degree of uneveness which can be regarded as a series of peaks and troughs. Therefore, to etch through the material to leave an exposed pattern of the substrate without any residue of the material being etched usually requires the etch process to be continued after initial exposure of the peaks on the surface of the substrate. The substrate itself is therefore also etched in the etching process. In many cases this may be undesirable as the substrate surface may have been provided with a very thin coating and therefore the etch process must necessarily be very carefully controlled to ensure that over etching does not occur. Additionally, extensive research is now taking place into the use of semiconductor organic materials in electronic devices, for example display devices, incorporating electroluminescent organic polymer light emitting diodes or integrated circuits incorporating organic polymer transistors. Conventional known etching processes present even greater concerns for the fabrication of such devices, as will be outlined below.
Electroluminescent displays represent a novel approach for fabricating high quality, multicolour displays. In an electroluminescent display a soluble polymer is deposited onto a solid substrate, such as for example, glass, plastics or silicon. Inkjet printing techniques have been proposed to deposit the soluble polymers not only because of the relatively low cost of such techniques but also because of the ability to use inkjet technology for large area processing and, therefore, the fabrication of relatively large area displays. For a multicolour electroluminescent display a number of soluble organic polymers may each be deposited as an array of dots of the polymers to provide the red, green and blue emissive layers for the display. The use of an inkjet technique makes this deposition of different polymers possible without deterioration of the polymer materials caused by the patterning processes.
Generally, two kinds of driving scheme can be used to address the pixels of the display. One is a passive matrix and the other is an active matrix scheme. The active matrix has patterned anode pixels, each with thin film driving transistors (usually two per pixel as the organic polymers are current driven devices) and a common cathode. In order to fabricate the patterned anode pixels and the thin film transistors (TFT's), conventional photolitographic technology is generally used. This process is carried out before the deposition of the organic layers so it does not affect the performance of the organic polymer materials. Fine patterning to fabricate the cathode is not required as the cathode may be a conductive layer common to all of the pixels. Hence, the common cathode can be fabricated over the organic layers using an evaporation technique, with the use of a metal shadow mask to define the edge frame of the cathode.
The passive matrix driving scheme uses patterned anodes and cathodes arranged as mutually perpendicular row and column electrodes on either side of the organic polymer emissive layers. In terms of the deposition of a cathode, the active matrix scheme is easier to fabricate, but the active matrix scheme still costs more to produce than the passive matrix scheme due to the formation of the TFT's for each pixel. Hence, the preferred way of driving such a display is to use a passive matrix addressing scheme. However, there are significant technical difficulties associated with the patterning of the anode and, in particular, the cathode for such displays.
The anode may be fabricated directly onto the substrate prior to the deposition of any soluble organic polymer layer. The anode is usually fabricated from indium tin oxide (ITO), as this material is conductive and relatively transparent. The ITO layer is formed as a continuous layer on the substrate and is then patterned using a photolithographic process to provide the anode array. However, the photolithographic process requires the use of a photo mask. Whilst such photo masks are commonly used to fabricate the anode array, their use becomes increasingly difficult as the size of the display area increases because problems are encountered in maintaining the required accuracy of definition throughout all areas of the mask. For use with large area electroluminescent displays, the use of such photo masks becomes prohibitively expensive which negates the potential cost benefits arising from the use of the relatively inexpensive organic polymer materials.
With regard to the cathode, the patterning of the cathode for an organic polymer display gives rise to significant difficulties. The cathode must necessarily overlie a soluble organic polymer layer. The traditional photolithographic techniques cannot be used for the patterning of the cathode as the etchants used severely damage or degrade the underlying organic materials. Other techniques have, therefore, been proposed for cathode patterning, such as the use of a stainless steel shadow mask, but such masks lack the required resolution in the fabricated array. Furthermore, the use of pre-patterned mushroom shaped photoresist dividers has also been proposed but such dividers are costly to produce and, in view of their fabrication process, are not suitable for large area patterning.
It has also been proposed to pattern a shadow mask through the use of inkjet printing of inert polymers followed by a lift off step using an adhesive tape. However, such a process suffers from poor resolution and usually gives rise to an unacceptably high density of defects in the achieved cathode array.